ИСТИНА

Generating harmonics from gas in a two colour laser feld allows to access both odd and even orders and to increase the eciency especially at short wavelengths. Here we show that in the basic conguration, orthogonally polarized beams, the ellipticity can reach up to 70% for even orders. This unexpected behaviour is observed with dierent gases, and in dierent spectral ranges from H6 to H42. Theoretical investigations based on non-perturbative theory reproduce almost perfectly the experimental data. Combining high ellipticity and high intensity for harmonics at short wavelengths makes this simple conguration a perfect tool for studying matter.